Evolución paleoambiental de la formación Río Mayer, Cretácico inferior, Cuenca Austral, provincia de Santa Cruz, Argentina

Abstract

La Formación Río Mayer es parte del relleno inicial de la Cuenca Austral, la cual se ubica en el sector más austral de la Patagonia. El área de estudio se encuentra entre los lagos San Martín y Argentino (Provincia de Santa Cruz), donde se relevaron diez perfiles sedimentológicos de detalle en los que se describieron 18 facies sedimentarias. Estas facies responden a diferentes procesos sedimentarios, principalmente autigénicos, hemipelágicos y episódicos. A partir de la distribución de las facies y del origen del material fósil se reconocieron tres paleoambientes sedimentarios: marino de plataforma externa, marino de plataforma externa con influencia deltaica y prodelta. El análisis de la distribución espacial y temporal de las facies condujo a la elaboración de un esquema de evolución paleoambiental para el Cretácico Inferior de la Cuenca Austral en la zona estudiada. La depositación de la Formación Río Mayer, por encima de la Formación Springhill, en el área de la Seccional Río Guanaco, comenzó durante el Berriasiano. En el Valanginiano la transgresión generó los depósitos incluidos en la Formación Springhill en la región de Lago San Martín, mientras que en el intervalo Hauteriviano-Barremiano toda el área de estudio presentaba características de plataforma externa distal. Durante el lapso temporal Aptiano-Albiano, se produjo la instalación de un sistema deltaico (Formación Piedra Clavada / Kachaike) en el norte del área de estudio entre los lagos San Martín y Viedma. Para este intervalo temporal la Formación Río Mayer en las regiones de Lago San Martín y Estancia La Vega, representa un ambiente de prodelta. Paralelamente, en la región sur del área de estudio se produjo la sedimentación de los niveles incluidos en un ambiente de plataforma externa influenciada por flujos turbidíticos distales de frente deltaico. Finalmente durante el Albiano superior-Cenomaniano inferior se desarrolló una transgresión marina, que marca el comienzo de la etapa de antepaís de la Cuenca Austral. En la región de los lagos San Martín y Viedma, la transgresión está evidenciada por los depósitos litorales de la Formación Mata Amarilla, mientras que en la región de Río Guanaco es representada por los depósitos de la Formación Cerro Toro.

The marine deposits of the Río Mayer Formation are part of the initial filling of the Austral Basin, which is developed in the southernmost part of South America, including the south of Chile and Argentina (southwestern Patagonia, Santa Cruz and Tierra del Fuego Provinces, Figs. 1, 2). The study area is located between the San Martín and Argentino lakes in the Santa Cruz Province (Fig. 3). The Río Mayer Formation (Fig. 2) was studied in three main localities, where detailed sedimentary logging was undertaken (Figs. 4, 5), taking into account the lithology, the primary sedimentary structures, the palaeontological content, rock bodies geometry, their orientation and the hierarchy as well as the contacts with underlying and overlying units. Eighteen sedimentary facies were defined (Figs. 6, 7; Table 1), which were grouped into three facies association representing different sedimentary palaeoenvironments, like marine outer platform (AF 1), marine outer platform influenced by a deltaic system (AF 2) and prodelta (AF 3) (Fig. 8, Tables 1, 2). The facies of the outer platform palaeoenvironment corresponds to the rocks traditionally assigned to the Río Mayer Formation, according to the available bibliography, and deposited by hemipelagic and authigenic processes interrupted by episodic sedimentation. The deposits of the outer platform influenced by a deltaic system palaeoenvironment are similar to the above mentioned but presents continental fossils that indicate a deltaic source. Finally, AF 3 of a prodelta palaeoenvironment is dominated by sandy/heterolithics levels associated with turbidity currents and hyperpycnal flows originated in a delta front. The best and most complete exposures of the unit were observed in the Río Guanaco locality, where palaeoenvironmental interpretations were performed, allowing the division of the 350 m-thick unit in three sections (Fig. 9). The lower section is composed mainly by black shales with both tabular and concretional marls levels (AF 1). Trace fossils are not recorded in this section, but ammonites, belemnites and bivalves are frequent. It has been interpreted as accumulated in a distal platform, under anoxic conditions. The middle section is characterized by bioturbated black marls and shales, with wellpreserved Zoophycos ichnofacies (Richiano et al., in press), occurring in a distal platform depositional environment. Body fossils are frequent, especially belemnites. The upper section is composed of massive black mudstones with intercalations of very fine- to fine-grained sandstones and less frequent conglomerates. In this section both debris flows and distal low-density turbidity current deposits were identified. Zoophycos ichnofacies and Ophiomorpha rudis ichnosubfacies were recognized in association with molds of petrified wood containing Teredolites isp. (Richiano et al., in press). The presence of turbulent currents and large trace fossils suggest a more oxygenated conditions. At Lago San Martín and Estancia La Vega localities the most significant characteristic is the presence of prodeltaic deposits in the transition between Río Mayer Formation and Piedra Clavada formations (Fig. 9). To sum up, after the Upper Jurassic rifting stage of the Austral Basin, the Springhill Formation represents the initial infill of the extensional structures. After that, during the Berriasian (Fig. 10a), the levels of the lower section of the Río Mayer Formation started to accumulate in the Río Guanaco area. During the Valanginian (Fig. 10b), an acceleration in the transgression generated the accumulation of the Springhill Formation in the Lago San Martín area while in the Río Guanaco region the middle section of the Río Mayer Formation developed. Between the Hauterivian and the Barremian all the study area was under distal platform conditions (Fig. 10c). A major change in the sedimentary systems of the Austral Basin during the Lower Cretaceous is the development of the deltaic Piedra Clavada Formation (Fig. 10d). This event generated two different palaeoenvironments in the Río Mayer Formation. Prodeltaic deposits are present to the north, while sediments of distal platform conditions with highly frequent sandy levels prevail southwards. Finally a new transgression indicates the beginning of the Cerro Toro and Mata Amarilla formations in the middle Albian- lower Cenomanian (Fig. 10e).